1. Field of the Invention
The present invention relates to a coaxial cable and a multi-coaxial cable that are suitable for use in devices having a display or imaging device, such as a notebook computer, mobile phone, ultrasound diagnostic device, endoscope, CCD camera, etc.
2. Background Art
A need has arisen in recent years for such devices to be reduced in size and weight, and for increased data transmission speeds and higher capacities to be achieved. In these devices, it is necessary to reduce the occurrence of electromagnetic interference (EMI) between signals. This interference is caused by the electromagnetic waves emitted from peripheral devices and signal lines through which high frequency signals are transmitted.
FIG. 5 is a perspective view showing one example of a conventional coaxial cable in a state in which a sheath of the distal end portion thereof has been removed. The conventional coaxial cable 1 has an inner conductor 2 in the center, an insulator 3 covering the conductor, a binding tape 3a being wound around the insulator 3, and an outer conductor 4 and a sheath 5 being disposed coaxially on the external periphery of the binding tape 3a. To achieve a reduction in the diameter of the coaxial cable 1, the insulator 3 is made thin. A fluorocarbon resin having a relatively low permittivity (dielectric constant) in comparison with other resins is preferably used for the insulator 3. In addition, in the conventional coaxial cable disclosed in Japanese Patent Application Laid-Open No. 11-144533, the fluorocarbon resin is foamed to create a large number of minute bubbles and to further reduce the permittivity of the insulator.
FIGS. 6A and 6B are schematic views showing another example of a conventional coaxial cable having a structure wherein a yarn is wound around an inner conductor. FIG. 6A is a cross-sectional view taken along a perpendicular plane to the axis of the cable, and FIG. 6B is a cross-sectional view taken along the axis of the cable. In the conventional coaxial cable 1′, a yarn 7 composed of, for example, an insulating material such as PET is spirally wrapped around the external periphery of a central inner conductor 2′, and an insulating tube 6 is extrusion-molded on, or tape is wrapped around, the external periphery of the yarn 7 to form a core portion. An outer conductor 4′ is provided to the external periphery of the insulating tube 6 and the outer surface of the outer conductor is covered with a sheath 5′ to ensure protection. This coaxial cable 1′ is known to allow dielectric loss to be reduced by interposing an air layer 8 between the inner conductor 2′ and the outer conductor 4′ without filling the gap with an insulator (for example, see Japanese Patent Application Laid-Open No. 7-182930 (FIG. 3)).
When a foamed fluorocarbon resin is used as the insulator 3 of the conventional coaxial cable 1, a foamed fluorocarbon resin tape is wrapped around the outer surface of the inner conductor 2 to form the insulator 3. In this case, the adhesion between the insulator 3 and the inner conductor 2 is low, and the inner conductor 2 is prone to slip out from the insulator 3. Another problem is that the line speed for manufacturing cannot be made high. In addition, the conventional coaxial cable 1 also has low heat resistance, because adhesive-coated polyester tape, which is commonly used as the binding tape 3a, sometimes is contracted by heat, and the insulator 3 is sometimes exposed during the soldering process.
In the conventional coaxial cable 1′, the insulating tube is extruded around the yarn 7 to form an insulator, and the line speed therefore can be increased. However, when the insulating resin cannot properly spreads on the yarn 7 and the insulating tube 6 is thinly formed, the insulating tube 6 readily breaks to form pinholes at a portion in which the tube makes contact with the yarn 7. Short-circuiting then readily occurs between the inner conductor 2′ and the outer conductor 4′. In addition, the conventional coaxial cable 1′, in which the inside diameter of the insulating tube 6 is equal to the sum of the outer diameter of the inner conductor 2′ and twice the outer diameter of the yarn 7, has a disadvantage in terms of making the coaxial cable thinner.